The present invention relates generally to vehicles and methods involving pressurized liquid fuel systems and, more particularly, to such vehicles and methods involving plural fuel tanks.
It is known to provide pressurized liquid fuel systems for fuels such as propane and Dimethyl Ether (DME). The present invention relates generally to any such pressurized liquid fuel system, but will be described in connection with its application to a DME fuel system, except where otherwise indicated. DME shows substantial promise as a fuel for use in diesel and other engines. Its advantages include that it is sulfur-free and combusts with minimal particulate emissions, making it particularly attractive for meeting increasingly stringent emissions regulations.
DME has a low vapor pressure saturation curve. It will remain a liquid under moderately low pressure at ambient temperature conditions. For example, it would be a liquid at 20° C. and 5 bars. A DME fuel tank containing fuel at an elevated pressure would be considered a pressure vessel because the internal tank pressure will exceed ambient pressure. Pressure tanks or vessels designed today follow well established rules and guidelines to prevent bursting. The tank design involves selecting the proper material and thickness to meet the tensile strength required for the application, as well to provide necessary chemical resistance to the material being stored. In addition, pressure tanks incorporate a pressure relief valve that allows the excess pressure to be bled out of the container before the bursting pressure is reached, such as by dumping liquid on the ground or vapor into the atmosphere. The geometry of the tank design is also important, as curved surfaces are better than flat surfaces for evenly distributing stresses in the container. A vapor head space is normally used for storage of liquid in a pressure container. For liquid propane, 20% vapor head space is typically provided and, for liquid DME, 15% vapor head space is typically provided.
DME has an energy equivalent level of 1.88 gallons of DME to 1.0 gallon of diesel fuel. Larger tanks are therefore required to accommodate DME fuel than to accommodate diesel fuel containing the fuel providing equivalent energy. This is significant for fuel systems such as are used on vehicles such as trucks. For a truck that uses DME fuel, much larger tanks must be provided to go the same distance as a truck running on diesel fuel. Not only is it necessary to increase the volume of the equivalent diesel tank by a 1.88 multiplier, it is also necessary to allow for the expansion of the liquid fuel without exceeding the bursting pressure of the tank. Standard practice is to allow a 15% gas buffer zone above the liquid for expansion. For example, given a theoretical diesel fuel tank size of 100 gallons, for a tank containing an energy equivalent amount of DME, the volume is first multiplied by 1.88 so that at least a 188 gallon tank is required for the energy equivalent amount of DME. When a 15% expansion volume is added to this 188 gallons, a 216.2 gallon tank is required, i.e., 188+(0.15)*(188)=216.2 gallon.
The large increase in volume necessary to provide DME fuel tanks that store an equivalent amount of energy presents problems in terms of packaging such tanks on a conventional diesel powered truck that would ordinarily be provided with, e.g., a 100 gallon tank. Consequently, the inventor proposes using multiple DME fuel tanks on vehicles, including on vehicles that have traditionally only used a single fuel tank.
The inventor has recognized that the use of multiple pressurized fuel tanks in a fuel system such as is used on a vehicle presents an array of problems. For example, it is not uncommon for a vehicle to be parked with one side exposed to the sun and the other side in the shade. If one fuel tank is in hot sunlight and the other remains cool, different pressures may develop in the tanks. If, for any reason, pressure is higher in one tank than the other, there will be a tendency for fuel in the higher pressure tank to be pumped into the fuel circulation system to the engine with a disproportionately higher flow rate than the fuel in the lower pressure tank (assuming each tank has an identical fuel pump). At the same time, there will be a tendency for fuel returning from the fuel circulation system to flow more into the lower pressure tank, rather than the higher pressure tank. Consequently, the risk exists that the fuel levels in the two tanks could become imbalanced with one being over-filled while one may be emptied. This can lead to a situation where fuel is pumped from only one tank at a time, and where excessive energy is expended by one pump ineffectively attempting to pump against another pump in a tank at a higher pressure.
The inventor has recognized that, for the multiple tank arrangements to be used effectively and so that the pumps in the tanks do not work against each other and the return flow is evenly divided between the tanks, the resistance to flow must be the same for each tank. It is, therefore, desirable to provide structures and techniques to equalize pressure in a fuel system including multiple fuel tanks.
In accordance with an aspect of the present invention, a vehicle comprising a pressurized liquid fuel system comprises fuel system components including a circulation system through which pressurized liquid fuel is adapted to be circulated, a first pressurized liquid fuel tank arrangement including a first fuel tank, a first fuel pump, a first feed line connected to the circulation system, and a first return line connected to the circulation system, a second pressurized liquid fuel tank arrangement including a second fuel tank, a second fuel pump, a second feed line connected to the circulation system, and a second return line connected to the circulation system, and a balancing conduit open to the first fuel tank and to the second fuel tank at least one of above a maximum liquid level of the first fuel tank and a maximum liquid level of the second fuel tank and below a minimum liquid level of the first fuel tank and a minimum liquid level of the second fuel tank, wherein the first return line and the second return line are configured such that a pressure drop across the first return line from the circulation system to the first fuel tank is adapted to be the same as a pressure drop across the second return line from the circulation system to the second fuel tank.
In accordance with another aspect of the present invention, a method for operating a pressurized liquid fuel system is provided and comprises circulating pressurized liquid fuel through fuel system components including a circulation system, pumping pressurized liquid fuel from a first pressurized liquid fuel tank via a first fuel pump to the circulation system through a first feed line connected to the circulation system while also pumping pressurized liquid fuel from a second pressurized liquid fuel tank to the circulation system via a second fuel pump through a second feed line connected to the circulation system, returning pressurized liquid fuel to the first pressurized liquid fuel tank from the circulation system through a first return line connected to the circulation system while also returning pressurized liquid fuel to the second pressurized liquid fuel tank from the circulation system through a second return line connected to the circulation system so that a pressure drop across the first return line from the circulation system to the first fuel tank and across the second return line from the circulation system to the second fuel tank is the same, and balancing pressure in the first fuel tank and the second fuel tank by permitting flow between the first fuel tank and the second fuel tank through a balancing conduit open to the first fuel tank and to the second fuel tank at least one of above a maximum liquid level of the first fuel tank and a maximum liquid level of the second fuel tank and below a minimum liquid level of the first fuel tank and a minimum liquid level of the second fuel tank.
In accordance with another aspect of the present invention, a method for operating a pressurized liquid fuel system is provided and comprises circulating pressurized liquid fuel through fuel system components including a circulation system, pumping pressurized liquid fuel from a first pressurized liquid fuel tank via first fuel pump to the circulation system through a first feed line connected to the circulation system while also pumping pressurized liquid fuel from a second pressurized liquid fuel tank to the circulation system via a second fuel pump through a second feed line connected to the circulation system, returning pressurized liquid fuel to the first pressurized liquid fuel tank from the circulation system through a first return line connected to the circulation system while also returning pressurized liquid fuel to the second pressurized liquid fuel tank from the circulation system through a second return line connected to the circulation system, and balancing pressure in the first fuel tank and the second fuel tank by permitting flow between the first fuel tank and the second fuel tank through a balancing conduit open to the first fuel tank and to the second fuel tank at least one of above a maximum liquid level of the first fuel tank and a maximum liquid level of the second fuel tank and below a minimum liquid level of the first fuel tank and a minimum liquid level of the second fuel tank, and adjusting pressure drop across at least one of the first return line from the circulation system to the first fuel tank and the second return line from the circulation system to the second fuel tank so that pressure drop across the first return line from the circulation system to the fast fuel tank is different from pressure drop across the second return line from the circulation system to the second fuel tank such that pressure in the first fuel tank and the second fuel tank is balanced.